1. Field of the Invention
The present invention relates to an image signal sync detector for detecting spatial and temporal displacements of pixels, lines or frames of image signals before and after signal processing such as their digital compression or expansion.
2. Description of the Prior Art
Signal processing such as digital compression and expansion of an image signal is preceded by its conversion to digital form when it is an analog signal, whereas when the processed image signal needs to be provided in analog form, the processing of the image signal is followed by its conversion to analog form. In this course of operation there is a possibility that images of input and output signals are displaced relative to each other.
When the input and output image signals are both digital signals, too, filtering or like signal processing may sometimes cause similar spatial and temporal displacements of the output image from the input signal.
As a solution to this problem, it is customary in the prior art to add sync signals to the image signal outside the available image area. However, such an approach does not sufficiently meet the above-mentioned problem of spatial and temporal displacements of an image by signal processing.
FIG. 1 is a diagram showing, by way of example, waveforms of sync signals for synchronization of image signals in the conventional television signal depicted, for instance, in xe2x80x9cTelevision Image Information Engineering Handbook,xe2x80x9d edited by Institute of Television Engineers of Japan, page 93, FIG. 3.28, Nov., 30, 1990. The conventional TV signal contains, for image synchronization, horizontal and vertical sync signals inserted in areas different from the video signal area as shown. The insertion of such sync signals in areas outside the available image area does not completely exclude the possibility of spatial and temporal displacements of images between input and output image signals nor does it guarantee that the output video signal is not out of registration with the input signal; furthermore, no means is available for detecting and correcting the displacements of the image.
Incidentally, there is disclosed, for instance, in Japanese Pat. Appln. Laid-Open Gazette No. 79946/98 an image signal sync detector of the type that adds sync signals to the image signal outside the available image area as mentioned above; besides associated descriptions are given in Japanese Pat. Appln. Laid-Open Gazettes Nos. 205156/96 and 109781/92.
Moreover, the recent advanced digital signal processing power has allowed multi-dimensional digital signal processing in the time domain as well as in the space domain, and consequently the displacements of an image before and after signal processing becomes a serious issue. In broadcasting which is particularly likely to involve multi-stage signal processing, spatial and temporal displacements of an image will cause a fatal problem.
With such a conventional method for the synchronization of image signals as described above, displacements of the output image signal relative to the input image signal cannot be detected by the sync signal added to the outside of the available image area of the image signal; hence, the prior art cannot sufficiently deal with the problem of image displacements resulting from image signal processing. The reason for this is that the possibility of the output image signal being displaced relative to the input image signal cannot completely be prevented using the sync signal added to the outside of the available area of the image signal.
It is therefore an object of the present invention to provide an image signal sync detector which detects spatial and temporal displacements of pixels, lines or frames of image signals resulting from digital signal processing such as coding and decoding of them and, if necessary, corrects the displacements.
In an image signal sync detector according to a first aspect of the present invention: a marker inserting part adds a sync signal to an image signal to be subjected to signal processing (hereinafter referred to as a yet-to-be processed image signal) in its available area; a marker detecting part detects the sync signal from a processed image signal; and a displacement detecting part detects the displacement of the processed image signal, based on the sync signal added in the marker inserting part and said sync signal detected in the marker detecting part.
The above image signal sync detector permits accurate detection of displacements of pixels or lines of the image signal which are caused by such digital signal processing as image signal coding and decoding.
In the image signal sync detector according to the first aspect of the present invention, the marker inserting part adds a signal having edges in one or both of horizontal and vertical directions, as the sync signal, to the yet-to-be processed image signal in its available area.
This permits accurate detection of displacements of an image in one or both of horizontal and vertical directions.
In the image signal sync detector according to the first aspect of the present invention, each frame number is multiplexed in the sync signal which the marker inserting part adds to the yet-to-be processed image signal.
This makes it possible to specify the current frame number by extracting it from the detected sync signal; in the case of a moving picture, its displacements can be detected in time as well.
In the image signal sync detector according to the first aspect of the present invention, the marker inserting part adds plural lines to the yet-to-be processed image signal.
With this scheme, even if the marker detecting part fails to detect the sync signal using one line, it can detect the sync signal using another line, allowing more accurate synchronization control.
In the image signal sync detector according to the first aspect of the present invention, the marker inserting part inserts blank lines before and/or after the line with the sync signal inserted therein.
This permits more accurate extraction of the sync signal from the processed image signal.
In the image signal sync detector according to the first aspect of the present invention, marker inserting part stops the addition of the sync signal to the yet-to-be processed image signal upon detection of the sync signal from the processed image signal in the marker detecting part.
This inhibits the addition of the sync signal to the image signal after the detection of the sync signal in the marker detecting part, permitting the removal of the sync signal from the available image area of the image signal.
In the image signal sync detector according to the first aspect of the present invention, the marker inserting part stops the addition of the sync signal to the yet-to-be processed image signal when displacement information, which is obtained at the time of detecting the sync signal from the processed image signal in the marker detecting part, remains unchanged over plural rounds of detection.
This permits more accurate detection of the sync signal, and provides more accurate information for removing the sync signal from the available area of the image signal.
In the image signal sync detector according to the first aspect of the present invention, the marker detecting part detects the sync signal by performing pattern matching between signals in a candidate area and a sync signal pattern of the same pattern as that of the sync signal.
This allows ease in detecting the sync signal in the marker detecting part.
In the image signal sync detector according to the first aspect of the present invention, the marker detecting part detects the sync signal by performing pattern matching between binarized versions of signals in a candidate area and a binarized version of a sync signal pattern of the same pattern as that of said sync signal.
This permits substantial reduction of the computational complexity for the pattern matching without causing a significant decrease in the accuracy of the sync signal detection.
In an image signal sync detector according to a second aspect of the present invention, an image sync correcting part corrects the displacement of the processed image signal through utilization of the displacement information detected in the displacement detecting part based on the sync signal added in the marker inserting part and the sync signal detected in the marker detecting part.
This permits correction of displacements of pixels or lines of the image signal caused by digital signal processing.
In the image signal sync detector according to the second aspect of the present invention, the marker inserting part adds a signal having edges in one or both of horizontal and vertical directions, as the sync signal, to the yet-to-be processed image signal.
This permits accurate detection of displacements of an image in one or both of horizontal and vertical directions.
In the image signal sync detector according to the second aspect of the present invention, each frame number is multiplexed in the sync signal which the marker inserting part adds to the yet-to-be processed image signal.
This makes it possible to specify the current frame number by extracting it from the detected sync signal; in the case of a moving picture, its displacements can be detected in the time domain as well.
In the image signal sync detector according to the second aspect of the present invention, the marker inserting part adds plural identical sync signals to the yet-to-be processed image signal in its available area.
With this scheme, even if the marker detecting part fails to detect the sync signal using one line, it can detect the sync signal using another line, allowing more accurate synchronization control.
In the image signal sync detector according to the second aspect of the present invention, the marker inserting part inserts blank lines before and/or after the line with the sync signal inserted therein.
This permits more accurate extraction of the sync signal from the processed image signal.
In the image signal sync detector according to the second aspect of the present invention, the marker inserting part stops the addition of the sync signal to the yet-to-be processed image signal upon detection of the sync signal from the processed image signal in the marker detecting part.
This inhibits the addition of the sync signal to the image signal after the detection of the sync signal in the marker detecting part, permitting the removal of the sync signal from the available image area of the image signal.
In the image signal sync detector according to the second aspect of the present invention, the marker inserting part stops the addition of the sync signal to the yet-to-be processed image signal when displacement information, which is obtained at the time of detecting the sync signal from the processed image signal in the marker detecting part, remains unchanged over plural rounds of detection.
This permits more accurate detection of the sync signal, and provides more accurate information for removing the sync signal from the available area of the image signal.
In the image signal sync detector according to the second aspect of the present invention, the marker detecting part detects the sync signal from the processed image signal by performing pattern matching between signals in a candidate area and a sync signal pattern of the same pattern as that of the sync signal.
This allows ease in detecting the sync signal in the marker detecting part.
In the image signal sync detector according to the second aspect of the present invention, the marker detecting part detects the sync signal from the processed image signal by performing pattern matching between binarized versions of signals in a candidate area and a binarized version of a sync signal pattern of the same pattern as that of the sync signal.
This permits substantial reduction of the computational complexity for the pattern matching without causing a significant decrease in the accuracy of the sync signal detection.